That seems really interesting :o The dragon vein ability your avatar has could maybe be a part of that objective where you can maybe block off enemy units by removing platforms or maybe form platforms to run away. We don't know the limitations or extent of the dragon vein ability yet, so I'm just assuming at this point, but that would be pretty cool!
In Fire Emblem Awakening, there was an extra condition you had to clear to recruit a character named Donnel, where you had to level him up at least once in addition to routing all the enemies. So I was wondering if there might be more conditions like those in the new game, which were character specific?
Also, a fun objective I was thinking of was maybe "Open all the chests/pilfer all the items", where you have a map full of treasure chests and thieves and you have to somehow manage to protect the chests from enemy thieves, open them before they do, or attack and prevent them from running away with the items if they do open the chests before you do. I don't know how they could implement this objective into the story or if this objective might seem too impractical or silly, but it might be really fun since you would really have to micro manage your units well to clear the objective XD. In past games, I always had a hard time dealing with enemies on top of getting to chests before their thieves did and sometimes I would lose those chest items because I let the thieves run away and off the map lol.